An investigation was made of the mechanism responsible for the loss of glucagon-stimulated adenylate cyclase in poorly differentiated hepatomas. Adenylate cyclase activity and I125-glucagon binding were measured in the plasma membrane preparations over a wide range of glucagon concentrations. Neither glucagon-stimulated adenylate cyclase activity nor I125-glucagon binding could be detected in these plasma membranes. Detergent-solubilized membranes from these hepatomas also did not bind glucagon whereas the solubilized liver membranes bound up to 1.2 picomoles of I125-glucagon. In well-differentiated hepatoma membranes, adenylate cyclase activity and I125-glucagon binding were present. Fluoride and epinephrine-stimulated adenylate cyclase activity in all hepatomas was similar to that observed in normal liver membranes. We conclude that there is no detectable glucagon receptor in these undifferentiated tumors.